Weird deformation with subdivision surface
Take a look at the topology of your hand there. The entire part with the fingers is one big concave n-gon, not good. 

The ngon is what is making the subdivision surface freak out.

To fix this you need to change the topology of the fingers so that each finger is a separate face (example below)

 

Sadly there is no easy to fix for that. I tried using the knife tool and cutting in some additional faces, but that just moves the problem ngon, not get rid of it.